Field of the Invention
The present invention concerns: a method to acquire correction data in connection with pulse sequences for the acquisition of magnetic resonance measurement data whose echo times—the duration between excitation and measurement data acquisition of said pulse sequences—is less than 500 microseconds, as well as a magnetic resonance apparatus and an electronically readable data medium for implementing such a method.
Description of the Prior Art
Magnetic resonance (MR) is a known modality with which images of the inside of an examination subject can be generated. Expressed in a simplified form, the examination subject is positioned in a strong, static, homogeneous basic magnetic field (also called a B0 field) with a field strength from 0.2 Tesla to 7 Tesla or more in a magnetic resonance apparatus, such that the nuclear spins of the examination subject orient along the basic magnetic field. To trigger magnetic resonance signals, radio-frequency excitation pulses (RF pulses) are radiated into the examination subject, and the triggered magnetic resonance signals are detected and entered into data points in an electronic memory organized as k-space from the k-space data, MR images are reconstructed or spectroscopy data are determined. For spatial coding of the measurement data, rapidly switched magnetic gradient fields are superimposed on the basic magnetic field in the memory. The acquired measurement data are digitized and stored as complex numerical values in a k-space matrix. An associated MR image can be reconstructed from the k-space matrix populated with values, for example by means of a multidimensional Fourier transformation.
In comparison to computed tomography (CT) examinations, for example, MR examinations normally take a relatively long amount of time. Due to the longer duration, MR examinations are susceptible to patient movements during the measurement. Depending on the pulse sequence type that is used and the type of the examination, movements of the examination subject can create artifacts in the reconstructed image that can make a diagnosis impossible.
The contrast responses of CT and MR examinations are also very different. While MR examinations provide a very good soft tissue contrast, CT examinations are especially suitable for the measurement of solid substances such as bones. In most standard pulse sequences, such solid substances deliver no signal since the signal of a solid substance decays rapidly.
Pulse sequences with very short echo times TE—for instance TE less than 0.5 milliseconds—offer new fields of application here for nuclear magnetic resonance tomography (MR tomography). They enable the depiction of substances that cannot be depicted with conventional sequences such as (T)SE ((Turbo) Spin Echo) or GRE (gradient echo) since their respective decay time of the transversal magnetization T2 of these substances is markedly shorter than the possible echo times of the conventional sequences, and their signal has therefore already decayed at the point in time of acquisition. In contrast, with echo times in the range of these decay times it is possible to depict the signals of these substances in an MR image, for example. The decay times T2 of teeth, bones or ice lie between 30 and 80 microseconds, for example.
The use of sequences with ultrashort echo times (UEZ sequences) thus enables (for example) bone and/or dental imaging and/or the depiction of cryoablations by means of MR and positron emission tomography, and is usable for MR-PET (combination of MR and positron emission tomography, PET) or PET attenuation correction.
Examples of UEZ sequences are: UTE (“Ultrashort Echo Time”), for example as it is described in the article by Sonia Nielles-Vallespin, “3D radial projection technique with ultrashort echo times for sodium MRI: Clinical applications in human brain and skeletal muscle”, Magn. Res. Med. 2007, 57, Pages 74-81; PETRA (“Pointwise Encoding Time reduction with Radial Acquisition”) as it is described by Grodzki et al. in “Ultra short Echo Time Imaging using Pointwise Encoding Time reduction with Radial Acquisition (PETRA)”, Proc. Intl. Soc. Mag. Reson. Med. 19 (2011), Page 2815; or z-TE as it is described by Weiger et al. in “MRI with zero echo time: hard versus sweep pulse excitation”, Magn. Reson. Med. 66 (2011), Pages 379-389.
In these sequence types, usually a hard delta pulse is applied as a radio-frequency excitation pulse and the data acquisition is subsequently begun as quickly as possible by the hardware, or with an echo time of less than 500 microseconds between the excitation by the excitation pulse and the beginning of the data acquisition. In PETRA or z-TE, the gradients are already activated during the excitation. In UTE, the gradients are ramped up to the desired strength with the beginning of the data acquisition.
Sequences with ultrashort echo times are also movement-sensitive. Increased movement—and therefore more pronounced movement artifacts—can occur precisely given sequences with low noise development (such as in PETRA).
In general, in MR imaging various approaches are pursued in order to design MR examinations to be more movement-resistant. For example, it is sought to optimize the strategy of acquiring the measurement data by means of the pulse sequence, for instance via radial acquisition techniques or BLADE. Other methods attempt to supervising the (disruptive) movement, either via MR-based monitoring (for instance via intermediate scans or navigators) for localization of the examination subject or also via external sensors, for example with the aid of what are known as markers with which a movement during the examination can be externally observed. The knowledge obtained in such a manner about the movement during the examination can be utilized—either in post-processing steps (post-processing) of the measurement data and/or reconstructed image data—to make corrections, or also to detect portions of acquisition in which a significant movement took place, and to discard these and repeat them instead.
One approach for MR-based movement correction uses measurements (data acquisitions) known as FID (free induction decay) measurements, which are executed between the actual measurements of the measurement data for image reconstruction. Such an FID-based movement correction method is, for example, described in the article by Brau and Brittain: “Generalized Self-Navigated Motion Detection Technique Preliminary Investigation in Abdominal Imaging”, Magn. Res. Med. 55:263-270, 2008, or also in connection with multichannel coils in the article by Kober et al.: “Head Motion Detection Using FID Navigators”, Magn. Res. Med 66:135-143, 2011. Refocused, measured FID signals are thereby compared with a reference value. Depending on the clearance of the examination subject from the reception coil used for the FID measurement, a different intensity is measured. If the position of the examination subject has varied between the individual FID measurements, a different intensity is also measured in the individual FID measurements. A movement therefore can be detected, and specific measurements of measurement data for image reconstruction can be discarded and/or repeated on the basis of this movement information.